Development of downy mildew resistant soybean isoline 'Nakhon Sawan 1' through backcrossing

Soybean (Glycine max (L.) Merr.) var. 'Nakhon Sawan 1' (NS1) was developed for resistance to downy mildew disease incited by Peronospora manshurica (Naum.) Syd. The backcross method was used to transfer resistant gene(s) from var. 'AGS 129' to NS1. Six backcross generations, recurrent parent (NS1), and donor parent (AGS 129), were evaluated for 12 agronomic characteristics. They were days to flowering, days to harvest, plant height at flowering, plant height at harvest, leaflet shape, number of pods per plant, number of seeds per pod, seed width, seed length, seed thickness, 100-seed weight and seed yield per plot. Fifteen lines, with NS1 phenotype and expressed homogeneity in downy mildew resistance were visually selected from BC5F2 generation. The selected isolines were evaluated in a replicated trial, of which 4 characters viz. plant height, number of pods per plant, 100-seed weight and seed yield were recorded. The population mean of BC1 and later backcross generations were found not different from the recurrent parent in all traits, except for plant height at harvest and days to harvest. Which were stable since BC2 and BC6 generation, respectively. Seed yield was positively correlated with days to flower, days to harvest, plant height at harvest, leaflet shape, and number of pods per plant. One hundred seed weight was negatively correlated with days to flower, days to harvest, leaf shape, number of pods per plant and number of seeds per pod but positively correlated with seed width and seed length. Leaflet shape was positively correlated with the number of pods per plant, the number of seeds per pod and the seed yield but negatively correlated with seed width, seed length and 100-seed weight. All 15 isolines selected from BC5F2 were not different from NS1 in 4 traits studied. It was concluded that the backcross method employed to improve resistance to downy mildew on NS1 was successful in 5 backcrossing.